Two dams

I was asked the other day about the safety of living below Oakland’s dams. We have two of them, both of which I happened to photograph in March 2003 (back when I was still using film). Anthony Chabot built them both. This is the dam at Chabot Reservoir, at the very south end of Oakland. The Hayward fault is a couple hundred meters downstream.

This is the dam at Lake Temescal.

Both are earthen dams, basically massive piles of clay and boulders. The first was built in 1874-75 and the second in 1868. As the “great San Francisco earthquake” occurred on the Hayward fault in 1868, seismic safety was high in people’s minds. Here’s a page about their construction. These dams are generally considered sound and able to withstand another big one. Lake Temescal straddles the Hayward fault, but the dam is so massive and the water it holds so modest that even a 2-meter displacement on the fault will not lead to a dangerous failure, as I understand it.

The Calaveras Dam, farther south near Milpitas, is also of earthen construction. It crosses the Calaveras fault and is being replaced with a safer design; in the meantime the Calaveras Reservoir has been drained to half its volume.

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This entry was posted on 1 June 2009 at 10:32 am and is filed under Earthquakes, Oakland streams and water. You can follow any responses to this entry through the RSS 2.0 feed.
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It doesn’t happen. Earth dams can handle fault rupture much better than concrete dams can because earth dams can deform without cracking in two.

Depending on the magnitude of the fault rupture an earth dam may just deform a little but otherwise be fine, or may may develop cracks. Cracks aren’t necessarily bad for well-designed modern dams (like maybe 1970s-ish onwards) but can be bad for older dams because they can allow water to leak through them which will cause erosion which will widen the cracks and allow more water to leak through, accelerating the erosion.

Nowadays a well-designed earth dam will have filter layers on the downstream side that will trap eroded soil within the dam, effectively plugging any cracks that might form.

i wasnt talking about the dam itself cracking, but rather the fault that runs directly beneath the lake opening up and allowing the water to go ‘straight down’ as it were. like a sinkhole or something, though i understand that we’re not dealing with limestone terrain here. supposedly in the 1906 earthquake there were huge fissures in the ground near olema and of course displacement of 20+ feet, right?

I think fault ruptures that result in a huge crack opening in the ground are pretty rare, and one certainly wouldn’t be big enough or deep enough to drain a reservoir. It could provide a preferential flow path under the dam, but any dam designed with filters would also have them over the fault to prevent that from happening.

In short, I think it’s a lot more likely for a fault rupture to damage the dam than for a fault rupture to drain the reservoir into the ground like someone pulling the plug in a bathtub.

To answer your initial question, rob, that just does not happen. It’s an open question whether or not a space opens up somehow between the bottom of the dam and the ground beneath — that might allow a flood out. But the lake would not suddenly drain into the bowels of the Earth.

Where exactly is the dam at Temescal? I go there a lot and would love to check out Mr. Chabot’s handiwork. Thanks for the link on the construction of the dams, as well! I really look forward to posts on this blog.

it’s good to bring this to people’s attention. in teaching geology classes, i have only once, in 11 years, had a student who knew that Fremont stands to be annihilated by an earthquake-induced dam ruptures. most people simply have no idea about this, and are horrified when i show them the ABAG dam failure maps. :)

It’s actually extremely common for dams to be build over faults. The best place for a dam is a narrow, deep valley through which a bigger valley/depression drains, and a fault is the most likely thing to allow a narrow, deep exiting valley in that situation to form.

There may be a small fault under Chabot Dam but it probably isn’t considered active. I.e. there are no signs of a fault there having moved in the last 10,000 years.

James, there are faults and faults. Any fracture that offsets rocks qualifies as a fault, but there are no active major or minor tectonic faults, besides the Hayward fault, near Lake Chabot Dam. The ridge that buttresses the dam is a different rock type (the volcanic rock of Leona Quarry) from what surrounds it.

Yes, a new strand of the Hayward fault was recently mapped in the area south of the Chabot dam. Here’s the graphic from the Tribune; don’t expect the link to last more than a few months. It’s not really significant; it runs through uninhabited land and, being short, can’t store much seismic energy. The dam is very secure against earthquakes, as far as I can tell, because of its old-fashioned massive construction consisting of packed clay. If it did fail, that would be pretty awful.

Unfortunately, I can’t figure out where the California Geological Survey is hiding this new information.